This invention relates to a polishing apparatus, and particularly to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat and mirror-like finish.
In recent years, along with progress in the realization of highly integrated semiconductor devices, circuit wiring has been becoming finer and distances between wires have also been becoming smaller. In particular, in the case of sub -0.5 .mu.m photolithography, because the depth of focus is shallow, flatness of stepper focusing surfaces is required.
For this reason it is necessary to flatten the surface of the semiconductor wafer, and as one method of carrying out this flattening, polishing with a polishing apparatus has been being carried out. In this kind of polishing apparatus, a turntable and a top ring each of which rotate at an independent speed are disposed facing each other, the top ring applies a fixed pressure to the turntable and polishing is carried out with the workpiece held between the top ring and a polishing cloth on the turntable containing an abrasive liquid.
The polishing apparatus described above is required to perform polishing such that the workpiece after polishing has a high degree of flatness. For this reason, polishing apparatuses wherein the holding surface holding the semi-conductor wafer during polishing, i.e. the lower end surface of the top ring, and the contact surface of the polishing cloth making contact with the semiconductor wafer, and therefore the surface of the turntable to which the polishing cloth is affixed, have a highly accurate flatness have been considered preferable and have been used.
On the other hand, as factors influencing the polishing effect of a polishing apparatus, it is known that not only the shapes of the top ring holding surface and the polishing cloth contact surface but also the relative velocity of the polishing cloth and the semiconductor wafer, the distribution of the pushing pressure on the polishing surface of the semiconductor wafer, the amount of abrasive liquid on the polishing cloth and the time for which the polishing cloth has been used have an influence. Therefore, it can be supposed that if these factors were to be made equal over the entire polishing surface of the semiconductor wafer, a highly accurate flatness could be obtained.
However, among these factors influencing the polishing effect there are factors which can be made equal over the entire polishing surface and factors for which this is extremely difficult. For example, whereas the relative velocity of the polishing cloth and the semiconductor wafer can be made uniform by making the turntable and the top ring rotate at the same speed and in the same direction, it is difficult to make the amount of abrasive liquid uniform because of the effect of centrifugal force.
Therefore, with an approach which relies on making the factors influencing the polishing effect equal over the entire polishing surface, including making the upper surface of the polishing cloth on the turntable facing the lower end surface of the top ring flat, there is a limit to the flatness of the polished surface after polishing and there are cases wherein it is not possible to obtain the required flatness.
In this connection, a method for obtaining a highly accurate flatness, as shown in JP-A-6-333891 (Japanese Unexamined Patent Publication No. H.6-333891), includes making the holding surface of the top ring a concave or convex surface and distributing the pushing pressure over the polishing surface of the semiconductor wafer, thereby correcting nonuniformity of the polishing effect caused by dispersion in penetration of the abrasive liquid and the time for which the polishing cloth has been used.
Also, measures such as providing the top ring with a diaphragm structure and correcting nonuniformity of the polishing effect by changing the pressure distribution during polishing have been employed.
However, when the shape of the holding surface of the top ring is altered, because the holding surface of the top ring is always in contact with the semiconductor wafer it continuously has an affect on polishing throughout the polishing process. That is, there has been the problem that because the shape of the holding surface of the top ring tends to influence the polishing effect too much, it is extremely difficult to correct nonuniformity of the polishing effect by intentionally providing the holding surface of the top ring with a non-flat shape, and when the intended shape of the holding surface of the top ring is even slightly unsuitable, flatness of the polished surface of the wafer actually is lost or correction is insufficient and adequate flatness of the polished wafer surface is not obtained.
Also, when correction is carried out by altering the shape of the top ring holding surface, because the top ring holding surface is of substantially the same size as the polished wafer surface it has been necessary to perform complex shape correction in an excessively small area. This also has made carrying out correction of the polishing effect by altering the shape of the holding surface of the top ring problematic.
Furthermore, in conventional polishing apparatuses, and particularly in polishing apparatuses for polishing semiconductor wafers and the like, it is intended that the polished surface of the workpiece after polishing be flat. With respect to intentionally polishing to a non-flat shape or polishing so as to increase or decrease the amount of polishing of targeted areas of the polished surface, there have been almost no suitable means or apparatuses other than that described above.